The present invention relates to methods and apparatus for dynamically reconfiguring a pulse width modulation (PWM) approach for controlling an inverter driving a polyphase motor.
Polyphase motors, such as permanent magnet, synchronous machines must be driven such that the windings thereof are energized as a function of the rotor position in order to obtain a driving torque from the machine. The windings of the polyphase motor are typically driven utilizing an inverter driver that receives a direct current (DC) source of voltage and that produces an alternating current (AC) source of voltage for driving the polyphase motor.
In a three phase motor, the inverter driver circuit will typically utilize a six step commutation technique to ensure that the proper windings are energized as a function of the rotor position. The inverter driver circuit will also ensure that the proper levels of voltage and current are provided to the polyphase motor within a given step of the six step commutation sequence. This may be accomplished using a PWM method to chop the DC source voltage and regulate the current delivered to the windings of the polyphase motor. One PWM method that has gained wide use is the so-called chop-chop method, which may also be called the fast decay method. Another PWM method is called the chop-coast method, which may also be referred to as the slow decay method.
The chop-chop PWM method permits the polyphase motor to be controlled in all four quadrants of the torque versus speed curve. In other words, the chop-chop PWM method permits the polyphase motor to generate power, such as may be useful in regenerative breaking, returning power to the DC source, or both. Thus, the chop-chop PWM method is also sometimes referred to as a four quadrant control.
The chop-coast PWM does not permit the polyphase motor to generate power; indeed, it permits the polyphase motor to operate only in the first and/or third quadrant of the torque versus speed curve. The chop-coast PWM method has certain advantages, such as significantly reduced switching losses in the inverter driver circuit, reduced high frequency components of current in the motor windings, and reduced high frequency components of current in the DC source. The fact that the chop-coast PWM method operates only in the first or third quadrants of the torque versus speed curve creates some problems. The chop-coast PWM method does not permit the measurement of the motor current at all times with a single current sensor measuring current from the DC source. Thus, it is very difficult to control the motor current by measuring the current from the DC source while also achieving a wide bandwidth control. Another problem with the chop-coast PWM method arises when a sensorless commutation scheme is employed to monitor the rotor position of the polyphase motor. Sensorless commutation schemes measure the back electromotive force (BEMF) in order to monitor the rotor position of the polyphase motor. If rotor synchronization is lost while utilizing the chop-coast PWM method, unintended regeneration of power due to the polyphase motor inertia is fed back to the DC source, which is highly undesirable.
In order to avoid the undesirable problems associated with the chop-coast PWM method, designers have employed the chop-chop PWM method to implement single quadrant motor drive systems, despite the higher switching losses and increased high frequency components of current in the motor and in the DC source.
Accordingly, there are needs in the art for new methods and apparatus for driving polyphase motors, particularly where it is desirable to perform four quadrant operation, and also to enjoy lower switching losses, and reduced high frequency components of current in the motor and in the DC source.